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Punt TD., Riddoch MJ., Humphreys GW.

In bimanual coordination when moving the hands to two separate objects, subjects tend to initiate and terminate the movements together, even when the targets are at different distances or are of a different size. Additionally, each hand tends to scale its grasp independently to the object to be grasped. Here, we report the performance of a patient, who had previously shown signs of motor neglect, on two experiments investigating coupling and independence in bimanual coordination. The patient showed relatively normal bimanual behaviour for the transport phase of prehension when objects were placed at different distances (Experiment 1), but abnormal behaviour for the grasp component when objects were of different sizes (Experiment 2). Moreover, the contralesional limb demonstrated a dependency of grasp that was related to the object grasped by the ipsilesional limb. We discuss the possible underlying mechanisms of this dependency in relation to competitive motor programming and attentional bias. The results also reinforce the view that the transport and grasp components of prehension are distinct processes.